The invention relates to antiscatter grids used in radiological imaging.
A radiological imaging apparatus conventionally comprises a source of X-rays and an image receiver, between which the object of which it is desired to produce an image is positioned. The beam emitted by the source passes through the object before reaching the receiver. It is partly absorbed by the internal structure of the object such that the intensity of the beam received by the receiver is attenuated. The overall attenuation of the beam having passed through the object is directly linked to the distribution of absorption in the object.
The image receiver comprises an optoelectronic detector or a strengthening film/screen pair sensitive to the radiation intensity. Consequently, the image generated by the receiver corresponds in principle to the distribution of the overall attenuations of the rays due to the latter passing through the internal structures of the object.
Part of the radiation emitted by the source is absorbed by the internal structure of the object, the other part is either transmitted (primary or direct radiation), or scattered (secondary or diffused radiation). The presence of diffused radiation leads to degradation in the contrast of the image obtained and to a reduction in the signal/noise ratio. This is particularly problematic, in particular when it is desired to view details of the object.
One solution to this problem comprises inserting an “antiscatter” grid between the object to be X-rayed and the image receiver. These grids are usually formed from a series of parallel plates made of an X-ray-absorbent material. In the grids, called “focused” grids (according to the terminology defined by the IEC 60627 standard relating to “Diagnostic X-ray imaging equipment—Characteristics of general purpose and mammographic antiscatter grids”), all the planes of the plates intersect along the same straight line passing through the focal point of the radiation emitted by the source. Thus, these grids make it possible for the direct radiation to pass through and for the scattered radiation to be absorbed. These antiscatter grids have made it possible for the contrast of the images obtained to be considerably improved.
The conventional antiscatter grids comprise a series of oriented parallel plates comprising a material which strongly absorbs X-rays, such as lead, for example, held between strip inserts which are made of a material more transparent to X-rays than the plates, such as aluminum or cellulose fibers (paper or wood).
A conventional method of fabricating antiscatter grids comprises alternately stacking plates of absorbent material and strip inserts made of a material transparent to X-rays. In order to obtain a focused grid, the plates must be accurately positioned at a slight angle with respect to the previous plate, such that all the planes of the plates converge along a straight line passing through the source.
The disadvantage of a fabricating process of this sort is that it tends to create cumulative errors in the angular position of the plates. These errors are detrimental to the performance of the grid.